You can never compare an association with NULL in XPath, only attributes can be compared to NULL. You can use the following XPath constraint:
[Microflow_Case/Case[not(Case_Case/Case)]]
Yes, sure.
Use this constraint on your retrieve action (of Microflow objects):
[Module.Microflow_Case/Module.Case/Module.Case_Case/Module.Case = NULL]
Where Case_Case is the self reference from Case to Case (but I can't read the name of it in the image)